"This is Felix Felicis, I suppose? ...You haven't got another little bottle full of— I don't know —"
"Essence of Insanity?"
-- Hermione and Ron Weasley (HBP22)
When Harry took a dose of Felix Felicis and announced he was going to Aragog’s funeral because that would help him get Slughorn’s tampered memory for Dumbledore, Ron joked that he must have taken Essence of Insanity instead (HBP22). To Ron and Hermione, Harry sounded rather crazy at that moment.
We can probably assume that Ron was just joking and there is no potion actually called "Essence of Insanity." Maybe he was referring to "Confusing and Befuddlement Draughts" which "inflame the mind" (OP18). However, Felix Felicis seems to work in the opposite way from a Confusing Draught because instead of becoming more hot-headed and reckless, Harry stayed focused and calm.